1. Field of the Invention
The present invention relates to a buffer layer included in a photoelectric conversion device and a manufacturing method thereof, a reaction solution for use in manufacturing the buffer layer included in a photoelectric conversion device, a photoelectric conversion device having the buffer layer, and a solar cell using the photoelectric conversion device.
2. Description of the Related Art
Photoelectric conversion devices having a photoelectric conversion layer and electrodes conducting to the photoelectric conversion layer are used in various applications, including solar cells and the like. Most of the conventional solar cells are Si-based cells using bulk single-crystalline Si, polycrystalline Si, or thin film amorphous Si. Recently, however, research and development of compound semiconductor-based solar cells that do not depend on Si has been carried out. Two types of compound semiconductor-based solar cells are known, one of which is a bulk system, such as GaAs system and the like, and the other of which is a thin film system, such as CIS system formed of a group. Ib element, a group IIIb element, and a group VIb element, CIGS, or the like. The CI(G)S system is a compound semiconductor represented by a general formula, Cu1-zIn1-xGaxSe2-ySy (where, 0≦x≦1, 0≦y≦2, 0≦z≦1), and when x=0 the formula represents a CIS system, while when x>0 it represents a CIGS system. The CIS and CIGS are herein collectively denoted as “CI(G)S”.
Conventional thin film photoelectric conversion devices, such as CI(G)S systems and the like, generally include a CdS buffer layer between a photoelectric conversion layer and a translucent conductive layer (transparent electrode) formed thereon. In such a system, the buffer layer is normally formed by CBD (chemical bath deposition) method.
Roles of the buffer layer may include (1) prevention of recombination of photogenerated carriers (2) band discontinuity alignment (3) lattice matching (4) coverage of surface unevenness of the photoelectric conversion layer, and the like. For CI(G)S systems and the like, the CBD method, which is a liquid phase method, is preferably used in order to satisfy, in particular, the condition of (4) above, since the photoelectric conversion layer has relatively large surface unevenness.
In view of the environmental burden, Cd-free buffer layers are under study, and as a major component of Cd-free buffer layers, use of zinc systems, such as ZnO systems, ZnS systems, and the like is also under study.
Japanese Unexamined Patent Publication No. 2000-332280 (Patent Document 1) discloses a method of producing a Zn(O, OH, S) buffer layer using a reaction solution that includes a zinc-containing compound, a sulfur-containing compound, and an ammonium salt (claim 1). Patent Document 1 also describes that a reaction solution that includes 0.5 mol/l or less of ammonia is preferably used (claim 2). Patent Document 1 further describes that a reaction temperature of 10 to 100° C. and a pH of 9.0 to 11.0 are preferable (claims 6 and 7).
Japanese Unexamined Patent Publication No. 2001-196611 (Patent Document 2) discloses a method of producing a Zn(S, O) buffer layer using a reaction solution containing zinc acetate, thiourea, and ammonia (Example 3). In Example 3 of Patent Document 2, concentrations of the zinc acetate, thiourea, and ammonia are 0.025M, 0.375M, and 2.5M, respectively.
Japanese Unexamined Patent Publication No. 2002-343987 (Patent Document 3) discloses a method of producing a Zn(S, O, OH) buffer layer using a reaction solution which is a mixture of a solution provided by dissolving zinc salt in ammonia water or ammonium hydroxide water and an aqueous solution provided by dissolving sulfur-containing salt in purified water (claim 1). Patent Document 3 describes that the film forming is performed with a transparency level of the reaction solution of 100% to 50% (claim 1). Patent Document 3 further describes that a reaction temperature of 80 to 90° C. and a pH of 10.0 to 13.0 are preferable (claims 5 and 6).
Japanese Unexamined Patent Publication No. 2003-124487 (Patent Document 4) discloses a method of producing a Zn(S, O) buffer layer by a roll-to-roll process using a reaction solution containing zinc acetate, thiourea, and ammonia. In Example 2 of Patent Document 4, concentrations of the zinc acetate, thiourea, and ammonia are 0.025M, 0.375M, and 2.5M, respectively.
Japanese Unexamined Patent Publication No. 2002-118068 (Patent Document 5) discloses a method of producing a ZnS buffer layer using a reaction solution containing zinc sulfate, ammonia, and thiourea (claim 4). U.S. Pat. No. 7,704,863 (Patent Document 6) discloses a method of producing a buffer layer which includes the steps of dissolving a 0.05 to 0.5 mol/l of zinc sulfate and a 0.2 to 1.5 mol/l of thiourea in distilled water at a temperature of 70 to 90° C., adding about 25% ammonia in the amount of ⅓ of the water, and after the solution becomes transparent, dipping the substrate in the solution for about 10 minutes to maintain the temperature substantially at constant within the time (claim 1).
A literature by D. Johnston et al., “Chemical bath deposition of zinc sulphide thin films using sodium citrate as a complementary complexing agent”, Journal of Materials Science Letters, Vol. 20, pp. 921-923, 2001 (Non-patent Document 1) describes a method of producing a ZnS thin film using a reaction solution containing zinc sulfate, thiourea, ammonia, and sodium citrate. In Non-patent Document 1, the film forming is performed at a reaction temperature of 60 to 80° C.
A literature by H. J. Lee and S. I. Lee, “Deposition and optical properties of nanocrystalline ZnS thin films by a chemical method”, Current Applied Physics, Vol. 7, pp. 193-197, 2007 (Non-patent Document 2) describes a method of producing a ZnS thin film using a reaction solution containing zinc sulfate and thioacetamide. In Non-patent Document 2, the film forming is performed at a reaction temperature of 95° C. with a reaction time of 90 to 120 min.
When providing a buffer layer by CBD method, it is necessary to form a film that well covers an underlayer. Further, in view of the production efficiency and cost, a high reaction speed is preferable in the film forming process for the buffer layer by CBD method.
Japanese Unexamined Patent Publication No. 2007-242646 (Patent Document 7) discloses a method of forming a buffer layer by providing a nucleation site or a growing site, which is a particle of the same kind as or a different kind from that of the buffer layer, and forming the buffer layer with the nucleation site or the growing site as the starting point or the catalyst (claim 1). Further, ZnS is specifically cited as a major component of the particle serving as the nucleation site or the growing site and the buffer layer (claim 8).
As described in Patent Document 7, by growing a CBD film after forming a fine particle layer that functions as the nucleation site or the growing site of crystal growth, catalyst, or the like, the reaction speed of the CBD film forming process may be increased and a film well covering an underlayer may be formed stably by controlling the crystal growth through CBD reaction. This method, however, requires an additional process of forming the fine particle layer.
The present invention has been developed in view of the circumstances described above, and it is an object of the present invention to provide a Zn system buffer layer manufacturing method capable of forming a film well covering an underlayer at a practical reaction speed without essentially requiring a fine particle layer that functions as the nucleation site or the growing site of or catalyst for crystal growth.